Surface and submarine vessels are generally equipped with acoustic antennas for transmitting and/or receiving acoustic signals being propagated through the sea. An acoustic antenna is formed from a set of sensors (hydrophones) that can be accessed separately, i.e. the outputs of which may be combined in various ways.
The acoustic antennas thus comprise a set of hydrophones which may be arranged in the acoustic modules of the antenna in various combinations depending on the configuration of the antenna.
For example, a towed linear acoustic antenna (ALR or flute antenna) may be of a substantial length, measuring several hundreds of meters, and of small diameter. Such an antenna is towed by a towing cable of many hundreds of meters and is intended to be submerged up to quite substantial sea depths (the pressure increasing by one bar for every 10 m in depth). Owing to its length, such an antenna comprises a substantial number of hydrophones which are regularly distributed along the antenna in acoustic modules. It is known practice, for example, to arrange an elementary acoustic multisensor along the flute antenna in a separate acoustic module in order to remove the left/right ambiguity found in submarine acoustics. Each acoustic module is protected by a protective module, which is of small dimensions owing to the small diameter of the flute antenna. Each independent acoustic multisensor is mounted on a printed circuit board (concentrator system) at one end thereof and perpendicularly to the plane of the board. The acoustic module may additionally comprise multiple auxiliary pieces of equipment, such as depth and direction sensors, and pieces of electronic equipment which are mounted on the concentrator system, in the small space delimited by the protective enclosure of the acoustic module.
The acoustic multisensors are, in this case, designed to deliver electrical signals which are proportional to the dynamic pressures exerted thereon. However, their accelerometric sensitivity must be known so that the delivered signals are referenced according to terrestrial acceleration forces. This acceleration reference is obtained through the use of an acceleration sensor (also referred to as an accelerometer) mounted on the concentrator in combination with each acoustic multisensor and delivering a signal representative of the acceleration of the flute along two axes, and through compensation for the effect of this acceleration in a differential manner over the signals delivered by each acoustic multisensor. Such an accelerometer makes it possible to determine the biaxial acceleration of the object to which it is fixed for a horizontal antenna.
Such accelerometers are expensive. In particular, in the case of a flute antenna, the accelerometers are generally arranged in a known manner along the antenna, generally every 2 m for interpolation. Furthermore, for each acoustic multisensor, an accelerometer is provided on the same concentrator along with other electronic components of the acoustic module so as to avoid interpolation. Taking the length of the antenna into account, the number of accelerometers required and, consequently, the cost incurred by the set of these accelerometers may thus be very substantial.
Another drawback of these accelerometers is their bulk with respect to the volume of the enclosure of the acoustic module (itself limited by the small diameter of the flute antenna). In particular, it may be useful to stack multiple printed circuit boards and to integrate them within each acoustic module by connecting the acoustic multisensor to each of the stacked boards, thereby making it possible to ensure the continuous operation of the multisensor in the event of failure. However, the current bulk of conventional accelerometers (in the direction perpendicular to the axis of the antenna) and their number would not allow such a stack.